Will Phase 3 ever include modified current campaigns, or custom campaigns? Will there ever be a Polish campaign? are there any mods, phase 3 included that allow me to play as the polish in custom battles?

Currently, there is only 1 scenario in which you play against the Polish. You can create your own maps with Polish units on your side or as the enemy by using the editor. There are numerous Polish infantry and mechanized units available in Phase 3 in the Editor's Units>Polish category.